This invention relates to a conductive powder having high conductivity stability even at elevated temperature and a process for preparing the same.
When rubber and plastic articles are used in an electronic application where conductivity is often required, it is a common practice to mix conductive powder in the rubber and plastic. Metal powders such as copper powder, nickel powder and silver powder are often used as the conductive powder although they add to the weight of electronic materials because of their high specific gravity in the range of 8 to 11. This is undesirable because the electronic materials are desired to be lightweight. Carbon powders such as graphite and carbon black are also used as the conductive powder although they fail to provide electronic materials with a resistivity of 10xe2x88x923 xcexa9-cm order because of their own high resistivity.
Of the conductive powders, metallized powder particles having high conductivity which are prepared by coating insulating powder particles or low conductivity powder particles with a metal have advantages including an increased freedom of choice for the powder material serving as a nucleus. They find a wide range of application as a conductive filler or a component added to conductive adhesive and anisotropic conductive film. A number of processes have been studied for the production of such metallized powder particles, with the process of using an electroless plating solution containing a metal salt and a reducing agent being industrially used in practice. For example, by covering surfaces of glass beads with a continuous silver coat, Potters Co. developed a conductive powder exhibiting the electric properties of silver despite a low cost and a low specific gravity. As is well known in the art, this conductive powder was marked in 1978 by Toshiba Barotini K.K. and used in a variety of applications.
However, silver has the problem that it is oxidized or sulfided to increase its resistivity during long-term storage in a hot humid atmosphere. There is a desire to have a conductive powder having electric properties unsusceptible to oxidation or sulfiding like gold. With respect to the nucleus particles, glass beads are inadequate in electronic materials requiring a high degree of reliability because the glass beads contain a considerable amount of ionic metals such as Na, K, Ca, Mg and Fe in addition to SiO2. The metallized glass beads have another drawback associated with working. When metallized glass beads are mixed with rubber or plastics, separation can often occur at the interface between silver and glass beads. This necessitates the use of solvents having fire hazard and the use of rubber rolls rather than metal rolls.
The separation at the interface between metal and powder particles is still the outstanding problem in the manufacture of metallized powder particles. Since the powder particles and the metal have different interfacial properties, the metal will separate from the powder particles due to changes with time or environmental changes (especially temperature changes), resulting in a reduced conductivity.
In order to prevent powder particle-metal separation and produce powder particles having a metal coating closely adhered thereto, the following processes have already been employed. For example, (1) powder particles are etched to introduce irregularities in their surface to increase the surface area for improving the metal adhesion. (2) Powder particles are treated with a silane coupling agent such as a monomeric silane, typically xcex3-aminopropyltriethoxysilane for improving the metal adhesion. (3) Powder particles are treated with an organic resin such as an epoxy resin for improving the metal adhesion. See JP-A 59-182961.
However, in method (1), the degradation of powder particles by etching is a problem. In method (2), powder particles will agglomerate owing to alkoxysilyl groups. In either case, no satisfactory metallized powder is obtained. In method (3), the organic resin with which powder particles are treated can be decomposed or degraded at elevated temperatures so that the conductive powder deteriorates its conductive properties. For example, where the rubber or plastic article which is required to impart conductivity is a highly heat resistant silicon polymer, that is, silicone rubber, this article cannot be used in the temperature range of 150 to 250xc2x0 C., under which service the silicone rubber itself fully withstands, because the conductive properties are degraded.
An object of the invention is to provide a conductive powder having improved heat resistance, conductivity and conductivity stability and a process for preparing the same.
It has been found that by treating powder particles of silica or the like with an organic silicon polymer, especially an organic silicon polymer with reducing effect, typically having Sixe2x80x94Si bonds or Sixe2x80x94H bonds in a molecule, to cover the particle surface with a coat of the silicon polymer, treating the powder particles covered with the silicon polymer coat with a solution of a metal salt in the presence or absence of a surfactant to deposit metal colloid on the surface of the silicon polymer coat, and treating the powder particles with an electroless plating solution to deposit a coat therefrom, there are obtained metallized powder particles of the structure that particle surfaces are covered with the silicon polymer coat, on which the metal is carried and the metal coat is formed. The process is inexpensive and simple. The adhesion of the metal coat is very strong. The metallized powder particles can find use as fillers and anti-fungus agents having conductivity and catalysis.
The metal plating layer can be formed as a multilayer metal layer including a first metal layer typically of nickel and a second metal layer typically of gold overlying the first metal layer. More improvements are then made in heat resistance, conductivity and conductivity stability.
In general, organic silicon polymers are very interesting because of their heat resistance, flexibility, and thin film forming capability as well as the metallic nature and electron delocalization of silicon as compared with carbon. In particular, silicon polymers having Sixe2x80x94Si bonds or Sixe2x80x94H bonds, especially polysilanes or polysiloxanes having hydrogen atoms directly attached to silicon atoms are known as polymers having reducing effect and used in a variety of applications. It is also known that both polysilane serving as a precursor to silicon carbide ceramic material and polysiloxane serving as a precursor to silicon oxide ceramic material can be heat or otherwise treated into an insulating material having high heat resistance.
We already found that when a substrate is treated with an organic silicon polymer having reducing action and dipped in a metal ion-containing solution, metal ions are reduced on the substrate surface whereby the resulting metal colloid is carried on the substrate. While utilizing this colloid formation as a catalyst, electroless plating is effected. Then, a substrate having a metal coat firmly adhered thereto is produced.
Utilizing the above-described nature of the silicon polymer, we have developed a powder exhibiting stable conductive properties even at elevated temperatures and a process for producing the same. Electroless plating on powder particles, especially substantially ionic metal-free powder particles, is enabled by previously forming a layer of organic silicon polymer having reducing effect on the powder particles. By the electroless plating, powder particles are coated with a first metal layer and then with a second metal layer. Especially when an oxidation resistant metal is used as the second metal layer, little or no loss of conductivity occurs even at elevated temperatures. Final heat treatment results in metallized powder particles featuring a strong bond between the particle base and the metal layer. The resulting conductive powder can be compounded in heat resistant rubber such as silicone rubber without losses of its properties, so that the resulting compound is useful as a raw material for manufacturing reliable connectors and gaskets.
We have further found that by treating particles of silica or the like with an organic silicon polymer having reducing effect to form a silicon polymer layer on the particle surface, treating the particles with a solution containing a salt of a metal having a standard oxidation-reduction potential of at least 0.54 volt, thereby depositing colloid metal on the silicon polymer layer, effecting electroless nickel plating and then gold plating on the particles, there is obtained a conductive powder in which an organic silicon polymer layer, a nickel-phosphorus alloy layer, and a gold layer are formed successively on the particle surface. When the electroless nickel plating involves electroless nickel plating in a first electroless nickel plating solution having a first phosphorus reducing agent concentration, and electroless nickel plating in a second electroless nickel plating solution having a second phosphorus reducing agent concentration different from the first concentration, the resulting nickel-phosphorus alloy layer has a phosphorus content which differs between inner and outer surface regions and especially which is lower in the outer surface region than in the inner surface region. The metallized powder has a firmly bonded gold layer and high conductivity, and is heat resistant enough to prevent the plating layer from separating even in heat treatment above 200xc2x0 C. The conductive powder can be compounded in silicone rubber etc., from which reliable, highly conductive rubber parts can be manufactured.
More specifically, we have found that in the manufacture of conductive powder, a nickel-phosphorus alloy layer having a high phosphorus content is more adhesive to powder particles of silica or the like whereas a nickel-phosphorus alloy layer having a low phosphorus content is more amenable to displacement plating with gold. By effecting electroless nickel plating initially at a low pH and in the presence of an excessive amount of reducing agent relative to a nickel salt concentration, the content of phosphorus in nickel can be increased to enhance the adhesion to silica. Thereafter, an aqueous solution containing a nickel salt, a complexing agent and a pH adjuster is replenished to the plating solution to raise the pH thereof whereby plating is effected in the presence of a short amount of reducing agent relative to the nickel salt concentration. The reduced phosphorus content in nickel facilitates gold plating. There is produced a conductive powder of the four layer structure consisting of particle base-silicon polymer-nickel/phosphorus alloy-gold and featuring tight adhesion therebetween.
According to a first aspect of the invention, there is provided a conductive powder in which an organic silicon polymer layer and a metal layer are successively formed on surfaces of particles. The metal layer preferably includes a first metal layer and a second metal layer.
According to a second aspect of the invention, there is provided a conductive powder in which a partially or entirely ceramic layer of organic silicon polymer and a metal layer are successively formed on surfaces of particles. The metal layer preferably includes a first metal layer and a second metal layer.
According to a third aspect of the invention, there is provided a process for preparing a conductive powder, comprising the steps of treating particles each having a surface with an organic silicon polymer having reducing effect to form a silicon polymer layer on the particle surface; treating the particles with a salt of a metal having a standard oxidation-reduction potential of at least 0.54 volt, thereby depositing a colloid of the metal on the organic silicon polymer layer; and thereafter, treating the particles with an electroless plating solution, thereby depositing a metal layer on the outermost surface of the particles.
According to a fourth aspect of the invention, there is provided a process for preparing a conductive powder, comprising the steps of:
(1) treating particles each having a surface with an organic silicon polymer having reducing effect to form an organic silicon polymer layer on the particle surface,
(2) treating the particles with a solution containing a salt of a metal having a standard oxidation-reduction potential of at least 0.54 volt, thereby depositing a colloid of the metal on the organic silicon polymer layer,
(3) effecting electroless plating on the particles with the metal colloid serving as a catalyst, to deposit a first metal layer on the outer surface of the organic silicon polymer layer, and
(4) effecting plating on the particles to form a second metal layer on the first metal layer.
The process may further include the step of heat treating the particles covered with the metal layer at a temperature of at least 150xc2x0 C. for converting at least a part of the organic silicon polymer into a ceramic.
In one preferred embodiment, the first metal layer is selected from among nickel, copper, silver, cobalt, tungsten, iron and zinc, and the second metal layer is selected from among gold, platinum, and palladium. More preferably, the first metal layer is nickel, and the second metal layer is gold. Then the desirable metallized powder has a four layer structure consisting of particle base-silicon polymer-nickel-gold. Gold is preferable as the second metal layer presenting the outermost surface of the conductive powder because gold has the highest conductivity among noble metals and does not undergo a rise of resistivity due to oxidation and sulfiding during long-term storage in a hot humid atmosphere. Nickel is preferable as the first metal layer because it is characterized by a low cost, corrosion resistance and appropriate hardness and serves as a stable underlying layer for bearing the second metal layer.